The application of clear top coats over basecoat paint layers is becoming increasingly, popular in the automotive industry. In such a color-plus-clear or basecoat/clearcoat system the pigmented basecoat is sprayed on the automotive panel and is overcoated with a reactive clear composition which crosslinks and hardens after being sprayed and dried. For example, the recent patent to Simpson et al U.S. Pat. No. 4,681,811, describes a transparent topcoat formed by spraying over the paint layer a liquid crosslinkable composition comprising a polyepoxide and a polyacid curing agent. According to the patent, this provides a composite with outstanding gloss and distinctness of image.
Simpson et al give no suggestion that any problems result from mixing the components of the clearcoat composition well in advance of spraying the mixture.
Another kind of clear top coat composition for application by spraying is described by Ambrose et al. U.S. Pat. No. 4,699,814. This contains a low molecular weight epoxy-functional polyurethane obtained by reacting an isocyanate with a hydroxy polyepoxide. A polyacid curing agent is used. Again no problem is indicated with mixing the components well before the mixture is applied.
Recently a new process for applying paints and clearcoats to automobile bodies and other three dimensional articles which yields finishes of outstanding quality has been described in Reafler U.S. Patent Application Ser. No. 116,426, filed Nov. 3, 1987. The new process involves manufacture of a stretchable plastic film, then coating one side of the film by laminar flow with a pigmented basecoat, thereafter applying a clearcoat over the basecoat and applying an adhesive layer to the other side of the film. The coating technique used for optimum quality involves the continuous laminar flow coating of a moving web of the plastic film by contact with a horizontally extending bead of the liquid coating composition. The latter is extruded from the slot of an extrusion coating hopper.
Extrusion coating hoppers have been used previously in precision coating of photographic goods, as disclosed, for example, in Miller et al, U.S. Pat. No. 3,206,323. They can produce coatings having exceptional smoothness, gloss and other good qualities. They are, however, more sensitive to problems with the feed composition than is the conventional spray painting method of applying clearcoats to automobile bodies. Thus, it has been found that the clearcoat compositions begin to increase in viscosity soon after the components are mixed. The extrusion hopper coating method may produce coatings of less than optimum quality if the viscosity of the feed composition increases substantially during the coating operation.
It has also been found that after the components of the clearcoat are mixed in the supply vessel, heterogeneous particles or slugs of polymer may begin to form. These tend to stick to the metal in and around the hopper exit slot and cause line defects in the coatings. Evidently neither of these phenonmena cause problems in the spraying of clearcoat compositions, possibly because the high shear of the spray nozzles lowers the viscosity of the coating composition.
According to Hayward, Reafler and Schuler, U.S. Ser. No. 189,090, entitled "Coating Process", filed May 2, 1988, as a continuation in-part of Reafler Ser. No. 116,426, the coating of reactive clearcoats on films through the slot of a coating hopper can be improved by mixing the components of the clearcoat shortly before the composition is fed to the coating hopper.